fix - no args function call with anthropic api

src/magentic/chat_model/function_schema.py

@@ -403,7 +403,8 @@ def strict(self) -> bool | None:
         return cast(ConfigDict, self._model.model_config).get("openai_strict")
 
     def parse_args(self, chunks: Iterable[str]) -> FunctionCall[T]:
-        args_json = "".join(chunks)
+        # Anthropic message stream returns empty string for function call with no arguments
+        args_json = "".join(chunks) or "{}"
         model = self._model.model_validate_json(args_json)
         supplied_params = [
             param

tests/test_function_schema.py

@@ -600,6 +600,10 @@ def plus_with_annotated(
     return a + b
 
 
+def return_constant() -> int:
+    return 100
+
+
 class IntModel(BaseModel):
     value: int
 
@@ -807,6 +811,13 @@ def plus_with_config_openai_strict(a: int, b: int) -> int:
                 "strict": True,
             },
         ),
+        (
+            return_constant,
+            {
+                "name": "return_constant",
+                "parameters": {"type": "object", "properties": {}},
+            },
+        ),
     ],
 )
 def test_function_call_function_schema(function, json_schema):
@@ -855,12 +866,26 @@ def test_function_call_function_schema_with_default_value():
         '{"a": {"value": 1}, "b": {"value": 2}}',
         FunctionCall(plus_with_basemodel, IntModel(value=1), IntModel(value=2)),
     ),
+    (
+        return_constant,
+        "{}",
+        FunctionCall(return_constant),
+    ),
+]
+
+function_call_function_schema_args_empty_string_test_case = [
+    (
+        return_constant,
+        "",
+        FunctionCall(return_constant),
+    ),
 ]
 
 
 @pytest.mark.parametrize(
     ("function", "args_str", "expected_args"),
-    function_call_function_schema_args_test_cases,
+    function_call_function_schema_args_test_cases
+    + function_call_function_schema_args_empty_string_test_case,
 )
 def test_function_call_function_schema_parse_args(function, args_str, expected_args):
     parsed_args = FunctionCallFunctionSchema(function).parse_args(args_str)